1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) technique, and particularly relates to a method for estimating a parameter depending on a subject by computation.
2. Description of Related Art
An MRI apparatus is a medical image diagnostic apparatus that causes nuclear magnetic resonance in hydrogen nuclei in an arbitrary plane that traverses a subject, and captures a tomographic image in the plane from a nuclear magnetic resonance signal (NMR signal; echo signal). Generally, the MRI apparatus applies a slice magnetic field gradient pulse that specifies an imaging plane, and simultaneously applies an excitation pulse that excites magnetization in the plane, to thereby obtain an echo signal generated at a stage where the excited magnetization converges. Here, in order to assign positional information to the magnetization, the MRI apparatus applies a phase encoding magnetic field gradient pulse and a readout magnetic field gradient pulse in directions that are orthogonal to each other in a tomogram plane between the excitation and the obtainment of the echo signal.
The excitation pulse and each magnetic field gradient pulse are applied based on a predetermined pulse sequence. Various pulse sequences are known according to their purposes. For example, there is a gradient echo (GE) pulse sequence for performing high speed imaging, or the like. In the GE pulse sequence, a predetermined pulse sequence is repetitively executed to measure echo signals. One echo signal is measured for one magnetization excitation, and the amplitude of the phase encoding magnetic field gradient pulse is changed for each excitation, so that echo signals necessary for obtaining one tomographic image are measured. Hereinafter, an operation of executing the pulse sequence to obtain an image is referred to as imaging.
In the GE pulse sequence, there is a phase compensated pulse sequence. The phase compensated pulse sequence is obtained by adding a magnetic field gradient pulse for setting a time integration value of a magnetic field gradient on each axis to zero to a normal GE pulse sequence. Thus, a pixel value of an image obtained in the pulse sequence also depends on a resonance frequency. The size of a flip angle of this pulse sequence is generally larger than that of the normal GE pulse sequence, and its phase is alternately reversed. Further, a repetition time TR is relatively short, which is about 5 ms.
As a method for obtaining an image by the MRI technique, there is a method for calculating a desired quantitative value for each pixel using plural images obtained by executing a pulse sequence under different scan parameters. The quantitative value that is an obtainment target includes a value (hereinafter, referred to as a subject parameter) depending on a subject, and a value (hereinafter, referred to as an apparatus parameter) depending on an apparatus.
An image in which the subject parameter or the apparatus parameter is used as a pixel value is referred to as a quantitative image or a map. The quantitative image is calculated using a signal function that determines the relationship between the scan parameter, the subject parameter or the apparatus parameter, and the pixel value. The signal function is analytically obtained and determined for each pulse sequence. Here, according to the imaging schedule, there is a case where the signal function is not easily analytically obtained, or a case where the signal function is analytically obtained but is extremely complicated so that the calculation of the quantitative image is not easy. With respect to such a pulse sequence, there is a technique that calculates a signal function by a numerical simulation to obtain a quantitative image (for example, see JP-A-2011-024926).